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Ferromagnetism in suspensions of magnetic platelets in liquid crystal

Abstract

More than four decades ago, Brochard and de Gennes proposed that colloidal suspensions of ferromagnetic particles in nematic (directionally ordered) liquid crystals could form macroscopic ferromagnetic phases at room temperature. The experimental realization of these predicted phases has hitherto proved elusive, with such systems showing enhanced paramagnetism but no spontaneous magnetization in the absence of an external magnetic field. Here we show that nanometre-sized ferromagnetic platelets suspended in a nematic liquid crystal can order ferromagnetically on quenching from the isotropic phase. Cooling in the absence of a magnetic field produces a polydomain sample exhibiting the two opposing states of magnetization, oriented parallel to the direction of nematic ordering. Cooling in the presence of a magnetic field yields a monodomain sample; magnetization can be switched by domain wall movement on reversal of the applied magnetic field. The ferromagnetic properties of this dipolar fluid are due to the interplay of the nematic elastic interaction (which depends critically on the shape of the particles) and the magnetic dipolar interaction. This ferromagnetic phase responds to very small magnetic fields and may find use in magneto-optic devices.

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Figure 1: Magnetic nanoplatelets.
Figure 2: Switching of ferromagnetic domains in an external magnetic field as seen by polarizing microscopy.
Figure 3: Magnetization curves of monodomain samples show the switching behaviour of magnetic nanoplatelets in ordered nematic suspension.
Figure 4: Time sequence of images showing a complete switching of a monodomain sample.
Figure 5: Dependence of the relaxation rate of director orientational fluctuations on external magnetic field in samples with different concentration of magnetic nanoplatelets.

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Acknowledgements

This work was supported by the Slovenian Research Agency (A.M. and M.Č., grant no. P1-0192; D.L. and M.D., grant no. P2-0089-4). We thank the CENN Nanocenter for use of the LakeShore 7400 Series vibrating-sample magnetometer.

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Authors and Affiliations

Authors

Contributions

A.M. designed the study and performed the experiments; A.M. and M.Č. interpreted results and wrote the paper; and D.L. and M.D. designed and synthesized nanoplatelets, and prepared the suspension of nanoplatelets in isotropic solvent.

Corresponding author

Correspondence to Alenka Mertelj.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Images of polydomain samples with different concentrations of magnetic nanoplatelets in an external magnetic field.

The director lies in the plane of the sample either parallel or perpendicular to the external magnetic field, B. P and A show directions of polarizer and analyser, respectively. The scale bar in the first image is 40 µm.

Extended Data Figure 2 Images of monodomain samples with different concentrations of magnetic nanoplatelets in an external magnetic field.

The director lies in the plane of the sample either parallel or perpendicular to the external magnetic field. The scale bar in the first image is 40 µm.

Extended Data Figure 3 Sequence of images showing domain growth.

The external magnetic field is slowly increased in a sample that was quenched in the absence of an external magnetic field. The direction of the external field is perpendicular to n. If the field is switched off, the initial dark field is obtained. However, if the field is switched on immediately or within a few hours, the formed domains are still visible. The concentration of the platelets in 5CB was 0.16 wt%. The scale bar in the first image is 40 µm.

Extended Data Figure 4 Sequence of images showing transition from a polydomain to a monodomain sample.

The external field is parallel to n. The white lines are the edges of domain walls. The concentration of the platelets in 5CB was 0.3 wt%. The scale bar in the first image is 40 µm.

Extended Data Figure 5 Sequence of images showing the complete switching of a monodomain sample.

The field is applied parallel to n and in reverse direction to the field that was used during the quench to nematic phase and then switched off. Travelling white lines are the surface domain walls, where the director rotates by π. The concentration of the platelets in 5CB was 0.16 wt%. The scale bar in the first image is 40 µm.

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Mertelj, A., Lisjak, D., Drofenik, M. et al. Ferromagnetism in suspensions of magnetic platelets in liquid crystal. Nature 504, 237–241 (2013). https://doi.org/10.1038/nature12863

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